Human mortality is predominantly related to atherosclerosis. Atherosclerotic stenoses are either treated with percutaneous transluminal angioplasty (balloon dilatation) or by-pass surgery. Today, 1.5 million revascularizations with these techniques are performed each year in the United States only. Approximately 40% of the patients experience a repeated narrowing within the first year due to restenosis or graft-stenosis, which in turn may induce a recurrence of organ ischemia with dramatic increased incidence of heart-infarction, amputation of legs and stroke. The cost in USA only for stenoses in grafts implanted in the legs is calculated to $100,000,000/year.
Graft-stenosis is due to intimal hyperplasia (IH). IH is characterized by migration and proliferation of smooth muscle cells followed by matrix deposition. IH can be regarded as an excessive response with scar tissue. Recent evidence has shown that hemodynamic, physical forces are the major contributors to the development of IH. Lowering of the shoving force exerted by the blood (shear stress) accelerates the development of IH in autologous vein grafts (Morinaga 1987), prosthetic grafts (Geary 1994) and in balloon-injured arteries (Bassiony 1998). Increased blood flow (increased shear stress) induces regression of established IH in grafts (Mattsson 1997). High variation in the level of shear stress may also increase the risk of IH (Nanjo 2006). Another hemodynamic factor of importance is turbulence. Increased turbulence raises the amount of IH (Fillinger 1989). The improved clinical handling of graft stenosis is therefore dependent on knowledge in both medical and physical sciences (Sarkar 2006).
Bypasses to treat stenoses are today implanted end-to-side to the artery (FIG. 2). This gives rise to a reduced shear stress at the “toe” and the “heal” of the connection sites, especially at the distal anastomosis (Ojha 1993; Ojha 1994) (FIG. 3). The development of IH is further supported by the fact that the suturing of the anastomoses co-localizes with the areas with low shear stress. The trauma imposed by the stitches in the vessel wall and the level of shear stress together induce cellular growth through different mechanisms, with IH to follow. Low shear stress will also be present at the division of flow in the recipient artery. Furthermore, the standard end-to side connection leads to a locally increased radius (FIG. 4). The level of shear stress decreases when the radius increases. The surgical procedure therefore leads to low shear stress and local induction of IH.
The standard by-pass graft also creates turbulent flow at the toe and the heal of the connection site. Turbulent flow is a known inducer of IH, (Fillinger 1989).
The end-to-side connection in bypass surgery faces other principal problems. It creates a bifurcation with a primary down-stream outflow and a secondary outflow. Since the artery has its given diameter, the two outflows have the same cross sectional area in spite of different need of blood flow. There is a splitting angle of 180 degrees between these “branches”. These two constraints are part of the boundary conditions of the problem addressed by the present invention.
An improved graft should therefore be able to provide a high shear stress with as low variability as possible along with as low turbulence as possible. This will reduce the induction of IH and improve graft patency. Further aims of an improved bypass should be to minimize the needed driving pressure difference between the ends of the graft. This results in increased ability for the blood to flow through the conduit in presence of stenoses distal to the bypass. The separation of flow should be anatomically separated from the trauma by the stitches imposed by the surgery. The inducers of IH, hemodynamic factors and trauma, will thereby not be present together at the crucial connection site of the bypass to the recipient artery.
WO 2006/100659 describes vascular prostheses in the form of forked tubes. The disclosure however fails to provide a description of the geometrical features needed for a vascular prosthesis which provides a sufficiently high shear stress with a sufficiently low variability along with a sufficiently low turbulence to reduce the induction of IH and improve graft patency.